Well known to the art is the base-catalyzed condensation of phenol and formaldehyde to produce liquid condensates, commonly referred to as "resoles". Curing of resoles to higher molecular weight, cross-linked thermoset resins proceeds with generation of heat and is accelerated by acid materials. In the presence of strongly acidic accelerators of the exothermic curing reaction and a source of blowing action, resoles cure rapidly to cellular phenolic resins.
Relative to cellular polyurethanes, phenolic foam possesses better inherent resistance to burn with an open flame and emits very low levels of smoke on heating. Notwithstanding these highly desirable characteristics, phenolic foam in general has been used in commercial practice primarily for general packaging purposes and to embed floral arrangements. For more widespread industrial application such as for wall and pipe insulation, roofing insulation and other construction purposes, it is desirable to provide phenolic foam having an acceptable combination of mechanical properties such as low friability and high compressive strength within a specified density range.
In addition to the high friability and low compressive strength generally associated with phenolic foam derived from conventional phenol/formaldehyde resoles, also to be reckoned with is the ability of phenolic foam to undergo flameless combustion when heated to its decomposition temperature. This phenomenon is commonly referred to as "punking" and may be likened to the glowing of charcoal briquettes. Punking of phenolic foam can be initiated at temperatures as low as 500.degree. C. or even lower. Once started, the hazardous punking or after-glow state is self-sustaining and sometimes foam temperatures as high as 1700.degree. C. are reached.
One approach to improvement of the mechanical properties and punking behavior of phenolic foam is the addition of special ingredients to the foam formulation such as particular blowing agents, surfactants and fillers. For example, the formation of non-punking foam is reported in U.S. Pat. No. 3,298,973 by the employment of a particular catalyst mixture of at least two acidic agents. The catalyst is a solid mixture of boric acid, or its anhydride, and an organic hydroxy acid in which the hydroxy group is on a carbon atom not more than one carbon atom removed from a carboxyl group such as, for example, oxalic acid. Punking of phenolic foam is also inhibited by the incorporation of certain organo-phosphorous compounds such as those described in U.S. Pat. No. 3,673,130.
Improvement in the punking resistance of phenolic foam by the use of a particular class of aralkyl-substituted polysiloxane oils is described in copending patent application Ser. No. 649,815, filed Jan. 16, 1976, in the name of Anthony J. Papa, now U.S. Pat. No. 4,070,313, granted Jan. 24, 1978. It was also found that the friability resistance of the phenolic foam could be improved by the use of the aralkyl-substituted polysiloxane oils in combination with polysiloxane-polyoxyalkylene copolymers of the type described in U.S. Pat. No. 3,271,331. However, as further described in the aforementioned patent, the improvement in friability was achieved at some sacrifice of the punking resistance otherwise achieved when the aralkyl-substituted siloxane oil is used as the sole surfactant component of the foam formulation.
Another approach to improving the mechanical properties of phenolic foam is to modify the resole during its preparation by the use of a combination of phenolic compounds. For example, in U.S. Pat. No. 3,639,303, it is reported that phenolic foam of improved toughness and abrasion resistance is realized when the resole is produced by the reaction of formaldehyde with a mixture of a mononuclear phenol and a polynuclear diphenol such as a mixture of phenol itself and a bis(p-hydroxybenzyl) diphenyl ether. Further, in U.S. Pat. No. 3,876,620 and a division thereof, U.S. Pat. No. 3,953,645, the formation of phenolic polymers by a sequential two-step polymerization process is described. In the first step, ortho-cresol and formaldehyde are reacted to provide an intermediate polymer which is then reacted in a second step with phenol and additional formaldehyde. Relative to foam derived from polymers produced with phenol and formaldehyde only, the patentee reports that foam derived from the ortho-cresol/phenol/formaldehyde polymers prepared in accordance with his claimed method, is of low friability. Indeed, in Table III at columns 13-14 of U.S. Pat. No. 3,876,620, the patentee reports that foaming of phenolic polymers produced by the claimed sequential two-step method, provided foam which exhibited only an 8 percent weight loss (ASTM C-421, 10 minute friability test), whereas the weight loss of foam derived from phenol/formaldehyde polymers which did not contain the presence of the difunctional ortho-cresol, provided "an undesirably high friability of 55%" (column 14, lines 26-29), as summarized below:
______________________________________ U.S. Pat. No. 3,876,620 ______________________________________ Foam Example 21 27 Phenolic polymer from Example No. 1 18 Mole ratio of o-cresol/phenol in polymer 1/1 0/1 Foam density, pcf. 1.7 1.6 Friability, 10 minute weight loss, % 8 55 ______________________________________
Although the patentee has provided a method for achieving phenolic foam of improved friability resistance, the method depends upon a two-step polymerization in forming the phenolic polymer and the use of ortho-cresol as an additional reactant which adds to the cost of polymer manufacture.
The aforementioned U.S. Pat. No. 3,876,620, filed Apr. 26, 1973, is also of interest from the standpoint of the following summary of the prior art set forth therein at column 1, lines 12-23:
"A number of the cellular materials described in these [i.e., the references noted at column 1, lines 6-12] and in other patents exhibit advantageous properties of compressive strength, self-extinguishing character, and low thermal conductivity. However, known cellular materials produced from phenolic polymers exhibit an undesirably high friability. This undesirably high friability is accompanied by increased dustiness and low tensile strength. In fact, this undesirably high friability has practically precluded the use of phenolic polymers as structural laminates such as wallboard. See also Modern Plastics Encyclopedia, Volume 41, pages 362-363 (1964)."
Moreover, although the teachings of U.S. Pat. No. 3,876,620 are directed towards providing phenolic foam of improved friability resistance, the patentee does not define other physical property measurements such as compressive strength and punking behavior, nor does the patentee deal with foam processability or overall foam quality such as the presence of blow holes or voids.
A third approach to improvement of the properties of phenolic foam is, of course, one which combines modification of the phenolic polymer during its preparation, with the use of special ingredients in the foam formulation. This approach is illustrated by U.S. Pat. No. 3,968,300 which emphasizes the use of ortho-cresol in the preparation of the phenolic polymer to be foamed and employs the use of a particular mixture of trichlorofluoromethane and 1,1,2-trichloro-1,2,2-trifluoroethane to give a foam having a lower thermal conductivity (k factor) than a foam produced with either blowing agent alone.
By way of observation it is noted that, even though the illustrative low k factor foams of U.S. Pat. No. 3,968,300 are based on ortho-cresol/phenol/formaldehyde polymers produced by the two-step sequential method of U.S. Pat. No. 3,876,620, the reported values for friability of the low k factor foams are higher than those reported in the U.S. Pat. No. 3,876,620. This observation is noted by way of further illustrating the difficulty of optimizing a particular property of phenolic foam by special additives to the foam formulation without adversely affecting another property.
The complexities involved in seeking to improve overall phenolic foam properties by the use of special formulation ingredients, is also apparent from the teachings of U.S. Pat. No. 4,033,910. The latter patent deals with the use of particular mixtures of methyl formate and 1,1,2-trichloro-1,2,2-trifluoroethane as a blowing agent in the formation of phenolic foam and with the different effects of such mixtures on foam cell size and friability. The nature of these effects, as explained in the patent, depends on whether in the preparation of the resole, the aqueous alkaline condensate has been treated by the addition thereto of neutralizing acidic compounds (e.g., sulfuric acid) thereby forming salts, or whether the condensate has been subjected to treatment with cation and anion exchange resins to provide resoles free of ionic species in accordance with the particular method described in U.S. Pat. No. 4,033,909.
In view of the above, it is believed evident that the state of the art is such to indicate that, unless an additional reactant such as ortho-cresol is used in the preparation of the phenolic polymer to be foamed or, unless special ingredients are added to the foam formulation, it is difficult to obtain from phenol/formaldehyde resoles, phenolic foam having a good balance of properties, such as, in particular, low friability and high compressive strength at an economic foam density, or foam having these desirable mechanical properties as well as a reduced tendency to punk.
It should also be appreciated that, in addition to improving the mechanical properties and punking behavior of phenolic foam derived from phenol/formaldehyde resoles, it is also important that the resole have adequate reactivity to provide foam within an economic density range and yet, the resole must not be too reactive. If the resole is too reactive, the exotherm during subsequent foaming is difficult to control and thus low density foam plagued with blow holes is obtained, that is, foam processability is unacceptable.
Compounding the complexities of phenolic foam technology is the difficulty of reproducibly manufacturing foamable phenol/formaldehyde resins with reasonable assurance that a particular desired combination of foam properties will be obtained upon foaming. This difficulty may be attributable to the well-recognized complex nature of phenol/formaldehyde resoles. Although the exact nature thereof has not been elucidated, it is generally recognized that phenol/formaldehyde resoles contain varying amounts of mono-, di- and trimethylolated mononuclear phenols (phenol alcohols) as well as polymethylolated polynuclear phenols. Precise characterization of the distribution of such molecular species or oligomers is further hampered by the usual presence in the resole of water and varying amounts of free phenol and formaldehyde.
It is, therefore, a primary object of this invention to provide a particular class of phenolic polymers, or resoles, derived from phenol and formaldehyde as the sole reactants added to the condensation reaction, which resoles are foamable to cellular products having a good balance of mechanical properties such as relatively low friability and high compressive strength within an economic density range, as well as acceptable processability from the standpoint of being free of deleterious defects such as an excessive number of voids, and severe stress cracks.
A more particular object is to provide phenol/formaldehyde resoles which are foamable to cellular products having a density from about 1.90 to about 3.70 pounds per cubic foot and which, within this density range, have a friability resistance of less than 35 percent weight loss (10 minute test), a compressive strength of at least 20 pounds per square inch, and acceptable processability.
Another object is to provide phenol/formaldehyde resoles which are characterized in a manner which allows for their manufacture on a reproducible basis from the standpoint of consistent performance as resins foamable to cellular products having the aforesaid desirable combination of mechanical properties and acceptable processability.
Another object is to provide phenol/formaldehyde resoles which are foamable to cellular phenolic products having the aforesaid characteristics and which, in addition, exhibit a reduced tendency to punk.
A further object is to provide cellular phenolic products having a density from about 1.90 to about 3.70 pounds per cubic foot and which, within this density range, have a friability resistance of less than 35 percent weight loss (10 test), a compressive strength of at least 20 pounds per square inch, and acceptable processability.
A still further object is to provide cellular phenolic products having the aforesaid characteristics and which, in addition, exhibit improved punking resistance.
Various other objects and advantages of the present invention will become apparent from the accompanying description and disclosure.